Generally, a polyimide film is formed from a polyimide resin. Such a polyimide resin is a highly heat-resistant resin prepared by subjecting an aromatic dianhydride and an aromatic diamine or an aromatic diisocyanate to solution polymerization, thus preparing a polyamic acid derivative, which is then subjected to a ring-closing reaction and dehydration at a high temperature so as to be imidized.
In the preparation of the polyimide resin, examples of the aromatic dianhydride may include pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA), etc., and examples of the aromatic diamine may include oxydianiline (ODA), p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), methylenedianiline (MDA), bisaminophenylhexafluoropropane (HFDA), etc.
Since a polyimide resin is a very strongly heat-resistant resin, which is insoluble and infusible, and is superior in terms of thermal oxidation resistance, heat resistance, radiation resistance, low-temperature characteristics, chemical resistance and the like, it has been utilized in a variety of fields including those of advanced heat-resistant materials, such as automotive materials, aircraft materials, spacecraft materials, etc., and electronic materials such as insulation coating materials, insulating films, semiconductors, electrode protective films for TFT-LCDs, etc.
However, a polyimide resin is brown- or yellow-colored, attributable to its high aromatic ring density, and thus has low transmittance in the visible light range, and also, is unsuitable for use in an optical member due to the high birefringence thereof.
In order to impart transparency to a polyimide having a deep brown and yellow color, a linkage group (—O—, —SO2—, —CO—, —CF3CCF3—) or a side chain having a relatively large free volume is introduced to the main chain, thus minimizing the formation of an intermolecular or intramolecular charge transfer complex, whereby transparency may be realized.
However, such a transparent polyimide film may have decreased heat resistance due to the introduced functional group. This is considered to be due to the charge transfer complex, and the film becomes transparent but its heat resistance is decreased. When heat resistance is decreased in this way, the transparent polyimide film is difficult to apply to advanced material fields including displays or semiconductors, which require high processing temperatures. To solve this problem, attempts have been made to polymerize monomers in the solvent after purification, but without any significant increase in transmittance.
U.S. Pat. No. 5,053,480 discloses the use of an alicyclic dianhydride component instead of an aromatic dianhydride. The formation of a solution or a film is improved in transparency and color compared to the purification method, but the increase in transmittance is limited and thus unsatisfactory transmittance results. Also, the thermal and mechanical properties are deteriorated.
U.S. Pat. Nos. 4,595,548, 4,603,061, 4,645,824, 4,895,972, 5,218,083, 5,093,453, 5,218,077, 5,367,046, 5,338,826, 5,986,036, and 6,232,428, and Korean Patent Application Publication No. 2003-0009437 disclose the preparation of a novel polyimide having improved transmittance and color transparency in the range within which thermal properties are not significantly deteriorated using a linkage group such as —O—, —SO2—, CH2—, etc., a monomer having a bent structure connected to an m-position rather than a p-position, or aromatic dianhydride and aromatic diamine monomers having a substituent such as —CF3, etc. However, the above polyimide has high birefringence.
Meanwhile, a conventional glass substrate is difficult to realize flexible properties and may be easily broken, making it difficult to use in real-world applications. To manufacture a thin lightweight substrate, a conventional glass substrate is coated with a polyimide material, after which the glass is separated, or it is formed on a polyimide film, in addition to the use of the thin glass substrate. When a colorless transparent polyimide film is applied to display fields, it may be utilized for display devices having various shapes, may exhibit flexible properties, and is thin, lightweight and unbreakable.
Therefore, a transparent polyimide, which is to be applied to display processes, needs superior thermal stability that may endure display processing, high mechanical properties for preventing the breakage thereof, and low birefringence to ensure a desired viewing angle.